The invention is directed to novel internally blocked organoborates having utility in the initiation of addition polymerizable monomers. The invention pertains also to adaptations for adhesives, with particular reference to polymerizable acrylic adhesive systems.
Typical conventional reactive acrylic adhesives utilize well known non-borate free radical initiators. U.S. Pat. No. 4,348,503 discloses acrylic adhesives initiated by a dibasic acid perester and a metal ion. U.S. Pat. No. 4,081,308 discloses an acrylic adhesive initiated with a saccharin salt and xcex1-hydroxysulfone. U.S. Pat. No 4,331,795 discloses an acrylic adhesive cured with hydroperoxide, thiourea and a metal salt. The non-borate initiated reactions for the above adhesives are inhibited by oxygen.
Organoborane free-radical initiation has been reported in the literature. See, for example, Bawn, C. E. H., Margerison, D. and Richardson, N. M., Proc. Chem. Soc., 1959, 397-398; Furukawa, J., Tsuruta, T., J. Polym. Sci., 1958, 28, 727-729; Dotty, P. M., Fouss, R. M., Mark, H., Overberger, C. G. and Smets, G. J. Polym. Sci. 1958, 33, 502-504. Two mechanisms of the oxygen-organoborane reaction have been postulated. Mirviss, S. B., J. Am. Chem. Soc. 1961, 83, 3051-3056; Davies, A. G., Roberts, B. P., J. Chem. Soc. B, 1969, 311-317; Allies, P. G., Brindly, P. B., J. Chem. Soc. B. 1969, 1126-1131. One mechanism involves the homolytic cleavage of the oxygen-oxygen bond to generate an alkoxy radical and a dialkyl boratoxy radical. The other major pathway is believed to involve homolytic cleavage of the alkyl-oxygen bond to form an alkyl radical and a dialkyl boraperoxy radical. The alkyl radical may react with more oxygen to produce an alkylperoxy radical, which may then react with organoborane to regenerate the alkyl radical and the borane peroxide. Thus one of the advantages in the use of organoborane initiated reactions is that atmospheric oxygen can be used as the source for the co-initiator in the formation of radicals.
Approaches to control the reactivity of organoboranes are known. E. Frankland reported the synthesis of triethylborane and its air-stable complex with a Lewis base, e.g., ammonia. Phil. Trans. Royal Soc. Vol. 152, pp. 167-183 (1863). The air-stable amine complex is believed to slow down the oxidation of organoboranes by blocking the borane open site for oxygen binding, which is the first step in the reaction of organoboranes with oxygen. The initiator can thus be stored in the blocked state and then de-blocked with an appropriate de-blocking or de-complexing agent, such as a stronger Lewis acid.
British Patent Specification No. 1,113,722 entitled xe2x80x9cAerobically Polymerisable Compositions,xe2x80x9d published May 15, 1968 discloses a polymerizable composition adapted as a structural adhesive and containing acrylic monomer(s) and an peroxide-activated triaryl, e.g., triphenylborane, complex with hydroxide, ammonia, benzene, or an amine. The polymerization is activated by heating or the addition of an acid. The resulting compositions are reportedly useful as adhesives.
Japanese Patent App. 69-100477 discloses a simple adhesive containing methyl methacrylate, tributylborane, and PMMA for use in the bonding of polyolefins or vinyl polymer articles. Excellent tensile shear strengths of over 1800 p.s.i. were reported. Acrylic adhesives polymerized with tributylborane and other trialkylboranes were reported during the 1970""s. (See, U.S. Pat. No. 3,527,737 to Masuhara, et al. and GDR Pat. No. 2,321,215 to Masuhara, et al.)
Two-part adhesives utilizing in one part trialkyl-, triphenyl-, or alkylphenylborane blocked with a primary or secondary amine and in the other part an organic acid or aldehyde are reported in U.S. Pat. No. 5,106,928, Can. Patent No. 2,061,021, U.S. Pat. No. 5,143,884, U.S. Pat. No. 5,310,835, and U.S. Pat. No. 5,376,746.
U.S. Pat. Nos. 5,539,070, 5,690,780 and 5,691,065 disclose polyoxyalkylenepolyamine-blocked organoborane initiator, with a nitrogen to boron ratio of 1:1 to 1.5:1 useful for adhesives to bond low surface energy materials such as polyolefins and polytetrafluoroethylene.
Organoborane alkoxide complexes are known but there is little reported literature. See, Ludman, C. J.; Waddington, T. C. J. Chem. Soc. A 1966, 1816-1819 and Angew. Chem. Int. Ed. Engl. 1972, 11, 48-49. A combination of potassium methoxide with triethyl borane resulted in the isolation of the hygroscopic white solid potassium triethylmethoxy borate(III). Similarly, the synthesis of tetraalkylborates, such as sodium tetraethylborate(III) are known. Honeycutt, Jr,. J. B., Riddle, J. M., J. Am. Chem. Soc. 1961, 83, 369-373. The present inventors have shown that these tetraalkylborates are effective as initiators for acrylic adhesives with or without a deblocking agent. The particular advantage of these initiators over amine-blocked boranes is their tendency to remain colorless after cure. In contrast, amine-blocked borane initiated acrylic adhesives yellow with time.
Hydroxy and/or alkoxy complexing agents for organoborane initiators are described in WO 01/32716, published May 10, 2001. The complexed initiator has the following structure: 
wherein R1 is alkyl C1-10, R2 and R3 are alkyl or phenyl and Cx is an alkoxide, or hydroxide and the value of v is selected so as to provide an effective ratio of oxygen atoms of the alkoxides and/or hydroxides to boron atoms in the complex. Mixtures of hydroxide complexes with alkoxide complexes are suggested. One alkoxide blocking group is illustrated as
((xe2x88x92)Oxe2x80x94R4)nM(m+)
where R4 is independently selected from hydrogen or an organic group, e.g. alkyl or alkylene, M(m+) is a group IA, IIA, or ammonium countercation, n is an integer greater than 0, and m is an integer greater than 0. An organoborane complex having the following structure follows from this: 
It has been observed that cured polymerizable adhesives initiated with amine-blocked boranes begin to turn yellow due to the presence of the amine group. In contrast, the decomposition products of alkoxy-blocked organoborates (alcohols) and alkyl-blocked organoborates (alkanes) do not undergo the unfortunate yellowing with time. The avoidance of discoloration of the cured bond line is important where a bond line must appear on a visually accessible surface of a bonded article for acceptable aesthetics.
There are numerous considerations in the replacement of mechanically fastened articles with adhesive bonding. It has been observed that conventional alkoxyborates have relatively poor air stability. For example when sodium ethoxotriethylborate(III) is exposed to air for 24 hours, the borate loses 46% of its mass and becomes inactive, unable to initiate the polymerization of methacrylate monomers. Air stability is important for meeting the practical problems in the commercial manufacturing and packaging of formulated borane-containing adhesive systems.
Another requirement for practical application of adhesives systems is the control of open time. Open time refers to the ability of an adhesive to cure providing strong bonds after exposure to air. Amine blocked boranes are known to have open times of a matter of minutes. A blocked boron-based initiator having an open time beyond a few minutes would be of industrial importance.
The inventors have discovered a class of 4-coordinate borates where at least two of the coordinates form part of a ring and have surprisingly good air stability enabling their use in packaged two-part polymerizable adhesives. As illustrated below, internally blocked borates exhibit stability in air of several days and retain an ability to cure methacrylate monomers. Moreover, the internally blocked borate initiators of the present invention remain colorless after curing, whereas conventional amine complexed borates are known to exhibit yellowing after curing.
In accordance with one aspect of the invention, novel 4-coordinate internally blocked borates are disclosed. The heterocycles have as part of the internal ring structure boron and oxa- or thia- moieties. These borates exhibit unexpected stability in the blocked state, ready reaction with de-blocking agents, and rapidly initiate polymerization in the unblocked state. The novel internally blocked heterocyclic borates have the structures (I) and (Ixe2x80x2): 
wherein for (I) and (Ixe2x80x2) X is oxygen or sulfur; when X represents oxygen, n is the integer 2, 3, 4, or 5; when X represents sulfur, n is the integer 1, 2, 3, 4 or 5; R1, R2, R3, R4, R5 and R6 are, independently, substituted or unsubstituted alkyl or alkylene groups containing 1 to 10 carbon atoms, substituted aryl groups having up to 7 to 12 carbon atoms, or unsubstituted aryl groups; R3, R4, R5 and R6 in (I) can be hydrogen; R1 and R2 in (I) and (Ixe2x80x2) can be part of a second unsubstituted or substituted cyclic borate; R1 and R2 in (I) and (Ixe2x80x2) can comprise a spiro ring or a spiro-ether ring; R1 or R2 together with R3 or R4 in (I) can be linked to form a cycloaliphatic ring; or R1 or R2 together with either R3 or R4 in (I) can comprise a cyclic ether ring; and M is any positively charged species with m being greater than 0.
In another aspect of the invention, two-part addition curing adhesives are provided where one part contains a 4-coordinate internally blocked borate according to (II) and (IIxe2x80x2) 
wherein for (II) and (IIxe2x80x2) X represents xe2x80x94CHR7xe2x80x94, oxygen or sulfur; n is the integer 1, 2, 3, 4, or 5. R1, R2, R3, R4, R5, R6 and R7 are independently selected from substituted or unsubstituted alkyl or alkylene groups containing 1 to 10 carbon atoms, substituted aryl groups having up to 7 to 12 carbon atoms, and unsubstituted aryl groups. R3, R4, R5, R6 and R7 can be hydrogen; R1 and R2 in (I), (Ixe2x80x2), (II) and (IIxe2x80x2) can be part of a second unsubstituted or substituted cyclic borate, or R1 and R2 can comprise a spiro ring or a spiro-ether ring; or R1 or R2 together with R3 or R4 in (I) and (II) can be linked to form a cycloaliphatic ring; or R1 or R2 together with either R3 or R4 in (I) and (II) can comprise a cyclic ether ring; and M is any positively charged species with m being greater than 0.
Internal blocking refers to the presence of boron being part of the internal ring structure bridged across at least two of the four boron coordinates or valences. A critical requirement is met in that the internally blocked borates according to (I), (Ixe2x80x2), (II) and (IIxe2x80x2) have considerable stability of initiator activity after exposure to air. Additionally, they provide a rapid rate of polymerization of addition polymerizable monomers upon contact with a deblocking agent and the presence of air. The rapid polymerization rate enables improved formation of structural adhesive bonds. The unexpected stability in the blocked state, ready reaction with de-blocking agents, and rapid initiation of polymerization in the unblocked state provide shelf stability so that adhesives can be packaged in conventional containers, for example multi-part or multi-chamber cartridges
In accordance with the invention there is provided a two-part oxygen-promoted adhesive, where a first part comprises a polymerizable component and an optional deblocking agent, an optional accelerator which is either another more reactive borane initiator or a non-borane initiator; and the second part comprises the internally blocked borate (II) or (IIxe2x80x2) and a carrier liquid. The adhesive preferably contains monomer and/or oligomer or polymer components in either or both of the first and/or second parts. The preferred optional accelerator is a non-borane containing free radical initiator.
In accordance with the adhesive methods according to the invention there is provided a method for adhesively bonding two substrates together to form a bonded composite, the method to form the composite comprising the steps of:
(a) providing a first substrate and a second substrate that is the same material or a different material from the first substrate;
(b) applying to the first and/or second substrate the following materials in a mixture:
(i) at least one polymerizable component;
(ii) an effective amount of an internally blocked organoborate (II) or (IIxe2x80x2);
optionally (iii) an activating or deblocking agent, optional accelerator and/or optional non-borane free radical initiator; and
(c) mating the first and second substrates with the components of step (b) therebetween; and
(d) allowing the at least one polymerizable component to polymerize, at ambient conditions or with the optional application of heat, whereby the first and second substrates are adhesively bonded together.
In another aspect the invention relates to applying a primer prior to a method of bonding comprising applying a solution of the internally blocked organoborate (II) or (IIxe2x80x2) in an inert organic solvent followed by treatment of at least one substrate with the polymerizable composition.